Liquid crystal displays ("LCDs") are commercially important articles. Such display devices are growing in popularity and utility over the last several years. LCDs operate by modulating external light, either the light from the surroundings (in the reflective mode) or the light from a backlight source (in the transmissive mode). Large displays such as, for example, the flat panel displays of newer televisions and computer screens, use backlight to ensure better performance. In order to have a bright screen, the backlight source has to be quite bright. This is because light from the source passes through several stages before reaching the LCD and tends to lose brightness in this process. Additionally, due to scattering effects light uniformity across the screen is lost. One way to avoid this problem would be to increase the size of the light source. However, this is expensive, increases power consumption and adds to the weight of the overall display system. Thus, there have been several attempts to improve brightness of the light without much loss during transmission.
A typical display system comprises a transparent light guiding plate between the light source and the LCD. There are additional layers such as diffuser, brightness enhancer, reflector and the like. The light source is typically a cold cathode fluorescent lamp ("CCFL") and the light pipe is edge lit. The back side of the transparent plate carries a white dot pattern for causing light scattering by interrupting the total reflection inside the transparent plate. The pitch of the dot pattern or the size of the printed dots is a function of the distance from the primary light source to get a uniform brightness all over the surface area. The light thus scattered inside the plate to achieve uniformity then goes through the diffuser (which hides the dot pattern) and the other layers prior to reaching and lighting the LCD. It is estimated that typically only about 10-20% of the original light from the CCFL reaches the LCD. Attempts to increase brightness have concentrated on improving the efficiency of light scattered within the transparent light guiding plate.
The dots printed on the transparent plate cause the light to scatter. Such scattering centers allow more light to gather internally and improve the efficiency for the light that comes out of the plate. Typically, polymers (such as polymethyl methacrylate, "PMMA") are used as transparent plate materials. The addition of scattering centers would be one way to improve the lighting efficiency and simplifying construction. However, care must be taken not to lose the spatial uniformity of lighting over the lightpipe, and not to lose the color fidelity of the CCFL as a result of non uniform scattering over the wavelength range. Otherwise, the color gamut of the CCFL will appear different in the exit light from the lightpipe.
A. Horibe et al, Journal of the Society of Information Display, Vol. 3 (4), 169 (1995) teach the use of polymethyl methacrylate containing centers of a polymer of different refractive index (e.g. polystyrene, polybenzyl methacrylate, poly-2,2,2-trifluoromethyl methacrylate) as light guide materials. The author report that the luminance from such plate is more than 50% brighter than that from conventional PMMA plate system. However that publication does not describe a method of obtaining spatially uniform scattering over the whole of the lightpipe.There is a continuing need for an improved backlight system for display applications.
Light diffusion plates are used widely for lighting applications, the purpose being to provide uniform diffuse lighting. Examples of applications include covers for lighting and also projection display screens. Japanese Patent 6-279604 (Mitsui Petrochemicals Limited, 1994) discloses scattering type cyclic olefin copolymers with fillers for light diffusion plate applications.
U.S. Pat. Nos. 5,521,797, and 5,093,765 disclose uniform lightpipes with printed dots on back surface; European Patent Application Number 95301273.9 (filed Feb. 28, 1995) and U.S. Pat. No. 5,528,720 disclose wedge light pipes, all using clear nonscattering type plastic.
Other disclosures of interest in this regard are U.S. Pat. Nos. 5,346,954; 5,108,857; 5,004,785; 4,963,624; 4,929,523; 4,000,216; 3,992,486; EP Applications 94305105.2 and 92100847.0 and Canadian Application 2,141,919.
It is an object of the present invention to provide an improved backlight system.
It is an additional object of the present invention to provide a backlight system with enhanced brightness and uniformity of colors across the source wavelength range.
It is an additional object of the present invention to provide a backlight system with spatially uniform lighting over the whole length and width of the backlight.
It is an additional object of the present invention to provide a lightpipe material that is mechanically tough and has impact resistance.
It is an additional object of the present invention to provide a light diffusion screen which scatters light uniformly with wavelength and which is mechanically tough
It is a further object of this invention to provide a backlight system that is simple to construct using known polymer processing techniques.
These and other objects of the invention will be apparent to those skilled in the art from the following description and examples.